[PATCH v3 2/2] libtracecmd: Support changing /proc/kallsyms

[Ilya Leoshkevich <iii@linux.ibm.com>]

Running BPF selftests under trace-cmd intermittently fails with:

    error in size of file '/proc/kallsyms'

This is because these selftests load and unload BPF programs.
bpf_prog_put() uses workqueues and RCU, so these programs disappear
from /proc/kallsyms after a delay.

trace-cmd reads /proc/kallsyms twice: the first time to compute its
size, and the second time to copy it into the trace file. If the
resulting sizes don't match, which is what happens in this case,
recording fails.

Fix by updating the size. This will work even for sending trace data
over the network thanks to trace_msg_cache.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 lib/trace-cmd/trace-output.c | 38 +++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/lib/trace-cmd/trace-output.c b/lib/trace-cmd/trace-output.c
index c333553e..b981baf3 100644
--- a/lib/trace-cmd/trace-output.c
+++ b/lib/trace-cmd/trace-output.c
@@ -577,6 +577,39 @@ __hidden int tcmd_out_update_section_header(struct tracecmd_output *handle, tsiz
 	return 0;
 }
 
+/*
+ * For files that can change while reading them (like /proc/kallsyms) the
+ * normal "size != check_size" can fail. That's because from the time the file
+ * size is determined and written into the output to the time the actual data
+ * is copied into the output, the size can change. In this case, the size that
+ * was stored in the output needs to be updated.
+ */
+static int update_endian_4(struct tracecmd_output *handle,
+			   off_t offset, int val)
+{
+	tsize_t current;
+	int endian4;
+
+	if (offset == (off_t)-1)
+		return -1;
+
+	current = do_lseek(handle, 0, SEEK_CUR);
+	if (current == (off_t)-1)
+		return -1;
+
+	if (do_lseek(handle, offset, SEEK_SET) == (off_t)-1)
+		return -1;
+
+	endian4 = convert_endian_4(handle, val);
+	if (tcmd_do_write_check(handle, &endian4, 4))
+		return -1;
+
+	if (do_lseek(handle, current, SEEK_SET) == (off_t)-1)
+		return -1;
+
+	return 0;
+}
+
 static int save_string_section(struct tracecmd_output *handle, bool compress)
 {
 	enum tracecmd_section_flags flags = 0;
@@ -1135,6 +1168,7 @@ static int read_proc_kallsyms(struct tracecmd_output *handle, bool compress)
 	enum tracecmd_section_flags flags = 0;
 	unsigned int size, check_size, endian4;
 	const char *path = "/proc/kallsyms";
+	off_t size_offset;
 	tsize_t offset;
 	struct stat st;
 	int ret;
@@ -1166,13 +1200,15 @@ static int read_proc_kallsyms(struct tracecmd_output *handle, bool compress)
 	}
 	size = get_size(path);
 	endian4 = convert_endian_4(handle, size);
+	size_offset = do_lseek(handle, 0, SEEK_CUR);
 	ret = tcmd_do_write_check(handle, &endian4, 4);
 	if (ret)
 		goto out;
 
 	set_proc_kptr_restrict(0);
 	check_size = copy_file(handle, path);
-	if (size != check_size) {
+	if (size != check_size &&
+	    update_endian_4(handle, size_offset, check_size)) {
 		errno = EINVAL;
 		tracecmd_warning("error in size of file '%s'", path);
 		set_proc_kptr_restrict(1);
-- 
2.50.1